Bridging biochemical, structural, and behavioral sciences to understand people
So the brain is just another organ. But how crazy is it that the part of our bodies and the part of science that we know the least about is the same part that controls everything we think, say, and do? It’s common to have a double-minded approach when it comes to the brain: on one hand, it consists of neurons that form into structures that interact using biochemistry and chemical signaling, but on the other hand, it is these neurons, structures, and biochemical interactions dictate how we think, behave, and feel. These two aspects, the physical structures and their dominion over thinking, acting, and feeling, are linked in that, if there is an imbalance in chemicals or an abnormality in a structure, the results in thinking, behaving, and feeling are directly affected. On this website, learn about both aspects: the physical and the behavioral parts of the brain, and see that the brain is not some mystical reason for how we think, act, and feel, but rather is just another organ.
The brainstem is located at the top of the spinal cord where the medulla connects to the pons. With control of many subconscious functions, such as breathing and swallowing, the brainstem plays a major role in the autonomic nervous system. In addition, the brainstem connects the spinal cord and brain by means of relaying messages between the two. Reticular formation is a group of neurons that connects to the cerebral cortex and thalamus with the purpose of carrying the messages for stimuli.
The spinal cord is a length of neural tissue that is enclosed in the vertebrae. Stretching from the lower back up to the brain, the spinal cord consists of segments of nerves with white and gray matter. With the purpose of meeting the most basic needs for survival, the brainstem has branches of nerves that diverge from the main cord and travel to muscles and organs throughout the body. One of the most prominent functions of the spinal cord is its involvement in the reflex arc, in which it receives signals from a harmful stimulus and creates a quick response to avoid danger, such as when a hot surface is touched and the hand is jerked away.
Found at the top of the brainstem, the function of the thalamus is act as a bridge for signals to cross to get to where they need to go. To do this, it takes sensory signals (except the olfactory system) and carries it to the the areas in the cerebral cortex that receive these signals. It also acts in the opposite direction in that it carries instructions for actions from the medulla and cerebellum to the brainstem to be dispersed throughout the body.
Taking up about 85% of the brain’s weight, the cerebrum as a whole is by far the largest structure of the human brain. By achieving functions such as higher level thinking, decision making, and the ability to create and carry out plans, this region of the brain is said to be the cause of humanity. Since this is such a large section of the brain, it is further split into four lobes, the frontal, occipital, temporal, and parietal lobes, and a layer of neurons over the top, the cerebral cortex. These regions are then responsible for sensory functions, motor functions, and association areas.
The cerebral cortex is the thin layer of neurons covering the cerebrum that folds, giving the brain its telltale wrinkled appearance. With a lot of the cerebral cortex being gray matter, this is the site of connections between neurons. In fact, the average brain has over 300 trillion synapses in this region. As a result, the cerebral cortex is responsible for its role in thinking and controlling perceptions and consciousness.
Split into the primary visual cortex and the secondary visual cortex, the occipital lobe specializes in interpreting vision. The primary visual cortex serves to take information taken in by the eyes and preserve the integrity of the image by keeping the proportion, space, and size accurate. The information is split so that everything on the left goes to the right hemisphere, activating neurons on the right side of the brain, and everything on the right side of the image goes to the left side of the brain, thus activating the neurons on the left. The secondary visual cortex consists of the neurons around the primary visual cortex and makes the image more specific by adding color and motion.
If vision is a projection of what is physically around someone and hearing is an auditory depiction, the temporal lobe is what takes that basic information and makes it something that would register with knowledge or memory. For example, the fusiform face area is the part of the brain that recognizes faces whereas other parts of the temporal lobe recognize objects, landscapes, images, or other sights. The primary auditory cortex has primary and secondary auditory areas which process sounds, including an area on the left side of the brain that takes heard words and sentences and processes them to give them meaning. Once these visual and auditory stimuli are processed, the hippocampus and amygdala, which are also located in the temporal lobe, stores them to memory so that, the next time a similar sight or sound is seen or heard, connections can be made from the previous memory.
The frontal lobe is the part of the brain that gives an adult his or her maturity. As a person grows, the frontal lobe develops to form organized circuits of neurons located behind the forehead. Behaviors that society expects of someone who is “mature” come from this region of the brain including the ability to concentrate, plan, and follow societal norms. One subregion is the prefrontal cortex, which has a large portion of the functions. For example, it is in charge of directing and maintaining attention with the added job of blocking out background stimuli which may distract concentration. It creates and causes the body to carry out plans, and provides the basis for reception of and actions based on empathy and social norms. The orbitofrontal cortex, located behind the eyes, helps define personality by playing a role in emotions and impulse control. Finally, the primary motor cortex causes physical action based on decisions made in the other regions of the frontal lobe by sending messages down the brainstem to the muscles.
The parietal lobe controls the continuation of the touch and sight senses. Stimuli from touch receptors are connected to the brainstem, which sends the message up to the primary somatosensory cortex, a strip of neurons that runs down the sides of the brain to receive touch stimuli. Receptors on this cortex correspond with where the stimulus touches, and parts of the body that are physically close are likewise close on the somatosensory cortex. Collectively, the entire body is represented with the somatosensory homunculus the ratio of amount of area dedicated to a particular region based on the level of sensitivity. For example, a sensitive area such as the lips would have more area than a less sensitive area, such as the leg. The other primary job of the parietal lobe is to establish spatial relationships. Although the eyes can create a projection of the objects around them, the parietal lobe is needed to process this and understand the concept of space that is open, space that is taken up, and the distance between objects.
With the right hemisphere of the brain controlling the left side of the body and the left side of the brain controlling the right side of the body, a connection is needed to ensure communication between the two hemispheres. This connection is made by the corpus callosum, which consists of axon fibers that cross the space between the two hemispheres. Impulses can travel across the corpus callosum to facilitate the transportation of information between the two sides of the brain.
The cerebellum is most commonly known as the part of the brain that enables balance and controlled motion. It is small, round, and extends off the back of the brainstem. In addition to coordinating movement, it houses certain cognitive processes that involve movement including planning, memory, and putting language into actions. Voluntary movement comes from collaboration with the pons to maintain balance and steadiness. The nervous system trains the cerebellum to work independently and without thought so that motion can be achieved while the brain is performing other tasks. Interestingly, all of the neurons throughout the cerebellum are identical, suggesting that all signals are identical, but the area that receives them is what differentiates the signals into different actions.
The limbic system is known as the animalistic part of the brain that functions to perform the basic needs for survival. Located between the hemispheres of the cerebrum, the system mainly consists of the amygdala and hypothalamus.
The hypothalamus is part of the animalistic section of the brain that is geared toward basic survival. Located just below the thalamus, the hypothalamus releases hormones to cause feelings according to what the body needs. For example, when glucose levels are low, a hormone will be released signaling that the person is hungry so that food will be eaten and glucose levels can return to normal. Other controls that are regulated include blood pressure, water levels, and temperature. In addition, the hypothalamus creates feelings of lust to drive reproduction so that the animal will produce offspring and pass on its genetics. The ability to regulate these functions comes from the ability of the hypothalamus to reward the body, also through hormones, so that it can have control over what needs are met.
The hippocampus is the part of the brain in charge of creating and storing memories. Defined as connections between neural matter to form associations, memories are created by networks of neurons connecting via dendrites to enable the travel of impulses so that two ideas, objects, or events are linked and can easily trigger one another. In addition to memories of events, the hippocampus is responsible for remembering where places and objects are located, a direct connection to the sense of sight.
The amygdala works with the hypothalamus, hippocampus, and cerebral cortex to create connections that relate directly to emotions with an emphasis on fear. Physically just a pair of neural clusters that make up a part of the limbic system, the amygdala associates an event, object, or other stimulus with some emotion that goes with it. Evolutionarily, the connections that pertain to fear are especially important because of the use of fear to protect an animal from dangers to survival.
The basal ganglia is a system made up of small structures located under the cerebral cortex with the purpose of assisting the thalamus by carrying information from the cerebral cortex to the motor centers of the brainstem. Another structure is the nucleus accumbens, a collection of neurons that release dopamine when something pleasurable is seen. This is known as part of the reward system as dopamine, a neurotransmitter brings what is identified as relief and positive feelings.
The nervous system plays a major part in the body’s function of moving, responding to stimuli, and maintaining homeostasis or a stable environment. It is a very general term and is broken down into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The PNS is further broken down into the Autonomic and Somatic Nervous Systems, and the Autonomic system is divided into Sympathetic and Parasympathetic functions. All of these systems function differently but are interdependent to keep the body working properly. A commonality is the use of neurons to carry messages between parts of the system, providing the cell communication that makes the system so efficient. When all these factors work together, the body is in full communication with itself.
Central Nervous System
The central nervous system consists of the brain, the spinal cord, and the nerves that connect them. The structures are isolated by a blood-brain barrier, blood vessels that have small gaps to let only a few materials through, as protection. The brain is the control center, where all stimulus information goes to and all instructions for movement or functions come from. The spinal cord runs down the body and primarily collects messages from the neurons in the body to send them up to the brain. Its other use is the reflex arc, in which a stimulus that is labeled as dangerous, such as touching a hot stove, goes to the spinal cord. Without going up to the brain to save time and prevent further damage, the spinal cord sends a response back, causing a path to go from muscle to spinal cord to muscle.
Peripheral Nervous System
The Peripheral Nervous System is all of the nerves in the body not found in the spinal cord or brain. Most are found in the muscles or organs so that messages can be delivered to maintain function or stimulate movement. For example, the legs can get messages via the PNS to start walking or the heart can be stimulated to beat faster when running. The PNS is connected to the CNS by the spinal cord, so impulses are carried down and away from the brain and are picked up by nerve cells of the PNS. From here, the PNS is further divided into the Autonomic and Somatic Nervous Systems.
Somatic vs Autonomic vs Enteric
The PNS is further divided into the somatic and autonomic nervous systems. The autonomic nervous system includes all of the subconscious actions and functions. For example, since you continuously breathe without telling your body to inhale to bring in oxygen to go to your muscles and then exhale waste carbon dioxide, breathing is an autonomic function. Somatic functions are the opposite, meaning that they require a message from the brain to tell them to happen. For example, if you want to walk across the room, you control this movement with the direction from your brain, telling your muscles to move your legs and carry you across the room. The enteric nervous system consists of all parts of the gastrointestinal system, working to break down food into usable energy. It works independently of the other nervous systems, but it both influences and is influenced by the somatic and autonomic nervous systems.
Sympathetic and Parasympathetic
Part of the function of the autonomic system is to respond to stimuli that require action by the body. Specifically, the sympathetic system responds when there is a danger, sending the body into fight, flight, or freeze mode. Triggering the release of epinephrine (adrenaline) from the adrenal glands, the sympathetic system increases heart rate and breathing rate, contracts blood vessels, and stops digestion to put all available energy into the fight or flight response. This feeling is the rush of adrenaline you feel when someone jumps out from behind you or a car speeds past where you’re standing, almost hitting you. The body is preparing itself to fight a predator or run away as a method of survival. This response, however, takes a great deal of energy that the body cannot sustain for long amounts of time, so the parasympathetic system functions to release norepinephrine (noradrenaline) to return body levels to normal. Therefore, in the release of this hormone results in slowed breathing and heart rate, dilation of blood vessels, and the resumption of digestion.
All of the structures of the brain and nervous system are made up of neurons.The neuron is the type of cell that makes up the structures of the neurological system. With the telltale shape, parts of the neuron include the dendrites, the cell body, the axon, and the synapse. The cell body is found at the center and contains the nucleus and organelles that control the cell. Protruding from one side is the network of dendrites, long strings that connect each neuron with up to 7,000 others and make the brain into one large circuit. Coming out of the other side of the cell body is the axon, which is a long strand covered in myelin that conducts impulses toward the next neuron. The synapse is found at the end of the axon and is the site where neurotransmitters are released and impulses leave the neuron. Three general types of neurons make up the system: motor neurons, sensory neurons, and interneurons. Sensory neurons travel from muscles to the brain via the spinal cord to deliver stimuli or other messages. Motor neurons send instructions from the brain to muscles and organs to coordinate movement and functions. However, motor and sensory neurons cannot interact, so interneurons have the job of delivering messages between the two.
Electrical Stimulation and Impulse Travel
Neurons at resting potential, meaning that there is no impulse travelling across them, are negatively charged due to the number of positive protons outside the neuron. The difference between the negative inside and positive outside is what enables impulses to be carried down the neuron. When an impulse comes down the axon, causing the neuron to fire, the cell is depolarized as a result of the exchange of positive sodium ions and negative potassium ions. As a result, the neuron turns positive and is in the stage known as action potential. The depolarization of the neuron makes it more permeable, and more sodium ions are allowed in as most of the potassium ions are repelled. The purpose of a neuron firing is to either carry the impulse and relay it to a connected neuron or to release neurotransmitter chemicals to cause a response. This way, impulses can quickly be carried along the circuits in the brain, enabling fast spread of information, and neurotransmitters can cause reactions to stimuli sent to the brain.
Once the neuron is depolarized, it must be polarized so that it can fire again. In order to do this, sodium potassium pumps in the cell membrane of the neurons start working to pump three sodium ions out of the cell for every two potassium ions pumped in. As a result, the neuron can return to its negative resting potential and can fire again when the next impulse comes.
Neurotransmitters are chemical messengers that signal throughout the brain. Stored in vesicles in terminal buttons at the end of axons, impulses go to these terminal buttons and signal the release of the neurotransmitters. From there, the neurotransmitters exit the neuron and are released into the synaptic cleft between the terminal junction of the releasing neuron and the dendrites of the next. Neurotransmitters bind to the dendrites of the next neuron. Each neurotransmitter has a specific shape that corresponds with the shape of its receptor, enabling a lock-and-key fit. From there, responses can start. Drugs and toxins can cause problems by mimicking the shape of the neurotransmitters and binding to the receptors, either stopping the expected reaction or amplifying it. The signal is ended by reuptake of the neurotransmitters, in which the neurotransmitters are taken back into the terminal buttons to be used for the next impulse, enzyme deactivation, in which enzymes destroy the neurotransmitters by changing the shape, or autoreception, in which the amount of neurotransmitter released is controlled and the neuron can stop when the signal is no longer needed. Some examples of neurotransmitters include acetylcholine, epinephrine, norepinephrine, serotonin, and dopamine. Acetylcholine controls messages that go between neurons and muscles that cause the muscles to contract. Epinephrine and norepinephrine work together in situations when danger is detected to prepare the body for fight or flight and then restore it to normal function. Serotonin controls emotions and mood, and dopamine is known as the brain’s reward system.
White and Gray Matter
White and gray matter is named for the literal colors of the neurons seen in the brain. The white matter refers to myelin that coats the axons of neurons. Known as a myelin sheath, this is beads of fat that go down the axon to facilitate fast conducting of neurons. The gaps between the beads of myelin are known as Nodes of Ranvier, and the negative impulses cannot travel within the axon because of the polarity of the myelin. Therefore, it jumps from node to node, enabling faster travel than if it had to pass through the entire axon. Keeping in mind that the brain is full of circuits, myelin acts as the “rubber” of wires. Gray matter refers to the cell bodies and dendrites that form connections throughout the brain to make a network of circuits that connect all of the millions of neurons so that messages can be sent to any part of the brain. These are the metal part of the wire.
The thing about emotions is that it’s impossible to name an emotion and point to a single part of the brain that controls it. Instead, different aspects of different structures contribute, and the network of neurons that enables communication throughout the entire brain leads to what we call feelings. The other thing about emotions is that the physical changes can be named, the feeling can be subjectively described, and beliefs and understandings can contribute to how a situation is perceived and the reaction, but, in the end, there is no objective definition of happiness, sadness, or any other emotion.
Three theories make up current understandings of emotions: the James-Lange theory, the Cannon-Bard theory, and the Schachter-Singer two-factor theory. None have been disproven, although the Cannon-Bard theory is most widely supported by scientists. The Cannon-Bard theory suggests that emotional feelings and physical reactions are two separate entities that come from different areas of the brain as the result of one stimulus. Since the feeling and the physical reaction are simultaneous, they are perceived as being together. The James-Lange theory says that actions are the cause of emotions. For example, when someone sees a spider, their heart rate increases, their pupils dilate, and their palms start to sweat. As a result of these physical changes, fear is the labeled emotion. The Schachter-Singer two-factor theory requires the viewing of the situation as a whole in which the combination of the physical response and the idea that a feeling should come as a result, called an emotion label, trigger the emotion.
Emotions in the Brain
Although no one part of the brain can be linked directly to a particular emotion, it is known that the amygdala is the receiving unit of sensory stimuli that trigger emotions, and the orbitofrontal cortex helps to distinguish them. Stimuli can take two paths to the amygdala: either they can go straight through the thalamus to the amygdala for almost instantaneous, surface-level associations, or they can go through the sensory cortex and then to the amygdala. This path allows for the stimulus to be better processed and evaluated. This way, the feeling has more meaning and can contribute to the making of memories by altering how the hippocampus receives it. Being part of the limbic system, the amygdala has a major role in the processing of stimuli, especially those that lead to fear. Emotions, depending on the stimulus, vary based on their level of intensity and the valence, or if they are labeled as pleasant or unpleasant. The amygdala specializes in identifying the intensity whereas the orbitofrontal cortex identifies the valence. Stimuli that present a danger are more likely to be more intense, meaning that the amygdala will have a stronger and faster reaction and emotion attached to it. These fears are often remembered because of the way it is presented to the hippocampus and the survival advantage in remembering situations that cause fear. Emotions with a high intensity are put into long-term storage by the hippocampus whereas those with lower intensity go to short term storage. From here, people can be conditioned to feel fear in response to a particular situation or stimulus by repeatedly presenting an image seemingly harmless and connecting a strong emotion, such as fear, with it. Because of the brain’s high capacity of storing emotions, the image will eventually be associated with fear, leading to the immediate triggering of the emotion in response to the image.
A lot of people attribute behavior with a mind, a soul, or a set of morals. For example, the saying “mind over matter” suggests that a mind controls how someone behaves and responds to a certain situation. If you think about it, though, where is the mind located? In the head? The heart? What is really dictating this behavior? According to science, there is no structure for a mind or a soul, leaving only the brain to control behaviors.
Chemicals Change Behaviors
Physical structures or changes in the brain dictate behavior. One way this happens is in the release of chemicals that result in certain feelings and therefore actions. For example, when the brain releases serotonin, the feeling that accompanies is labeled as “happiness.” As a result, you may smile, causing your eyes to squint, your cheeks to lift, and the sides of your mouth to wrinkle. On the other hand, lower levels of dopamine in the brain is labeled as depression, causing behaviors such as frowning, poor posture, and negative thoughts.
Structures Change Behaviors
Physical structures or changes in the brain dictate behavior. One way this happens is in the parts of the brain that control how your body responds to signals. For example, in a properly functioning body, the brain stem is an intermediate structure between the brain and the spinal cord that relays messages from muscles to brain and brain to muscles. However, for people whose brainstems are severed, this carrying of messages cannot occur. As a result, symptoms include a lack of ability to maintain a heartbeat, the need of a machine to carry out respiration, and eventually death as the body can no longer carry signals and perform basic functions. This is a very extreme example of a malfunction in a structure leading to a detrimental behavior.
Another famous example of brain structures altering behavior is that of Phineas Gage. Gage worked building railroads by blasting apart masses of rock with dynamite. When the dynamite exploded early, the pole he used to push it into the hole shot up and through his head, destroying most of his frontal cortex. Gage survived, but his behavior was affected because of the loss of function of the frontal cortex. Since that region of the brain determines concentration, the ability to plan, and the ability to conceptualize societal norms and social cues, Gage lost this, causing him to behave like a child with a short attention span, a major focus on the present without looking ahead, and impulsively unable to follow societal norms.
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